1. Field of the Invention
The present invention generally relates to a heating device, a fixing device and an image forming apparatus. More particularly, the present invention relates to a heating device, a fixing device and an image forming apparatus in which a capacitor thereof can have a longer life-span.
2. Description of the Related Art
Many image forming apparatuses, such as copiers, form images on recording media, such as plain papers and OHP (OverHead Projector) transparency sheets, in accordance with an electrophotographic manner because of advantages thereof on speedy image formation, image quality and costs. In such an electrophotographic manner, a toner image is formed on a recording medium, and the formed toner image is fixed by applying heat and pressure on the recording medium. As a fixing method, a heat roller manner is widely used for safety. In a typical heat roller manner, both a heating roller for applying heat with a heat generation member such as a halogen heater and a pressure roller disposed to face the heat roller integrally form a mutual pressing part called a “nip part”. During passage through the nip part, a toner is fixed on a recording medium by applying heat and pressure to the recording medium onto which a toner image is transferred.
In recent years, image forming apparatuses, such as a copier and a printer, are designed to save energy because of considerable attention on environmental problems. In order to reduce energy consumption of an image forming apparatus, it is indispensable to save electric power consumed for a fixing device to fix a toner on a recording medium. In a conventional approach, power consumption of a fixing device is saved during waiting time of the image forming apparatus. Typically, the temperature of a heat roller is kept at a degree lower than a fixing temperature during the waiting time, and when the image forming apparatus is used, the temperature of the heat roller is raised to an available temperature immediately so that a user does not wait for increasing the temperature of the fixing roller. In this approach, a certain level of electric power must be supplied to the fixing device even during idle time thereof, thereby consuming an extra amount of energy. In general, it is said that the energy consumption during the waiting time reaches 70% through 80% of a total amount of energy consumption of an image forming apparatus.
Consequently, there are increasing demands of developing an image forming apparatus that can realize reduction in an amount of energy consumption to be reduced during waiting time and save electric power required to run the image forming apparatus. It is desirable that no electric power have to be supplied to such an image forming apparatus during idle time thereof. However, if an image forming apparatus were designed to consume no energy during waiting time thereof, it would take a long time, for example, a few minutes to above ten minutes, to heat the heating roller to a temperature of about 180.degree.C. at which the image forming apparatus becomes available, because the heating roller, which is configured from a metal roller made of iron or aluminum, has a large heat capacity in general. If a user has to wait for such a long time until the heating roller is heated, the user may feel inconvenienced by the image forming apparatus. For these reasons, it is desired to design a heating method that can save as a large an amount of power consumption as possible, and on the other hand, restart an image forming apparatus from waiting time thereof as fast as possible.
In order to raise the temperature of a heating roller in short time, a simple approach to increase an amount of input energy per unit time, that is, to use a larger size of rated electric power, is considered. In fact, many of image forming apparatuses capable of high speed printing, which are called “high speed machines”, correspond to power supply voltage of 200V. In ordinary offices in Japan, however, the power supply of 10V and 15 A is usually available as a commercial power source. Accordingly, if such an image forming apparatus is installed in an office in Japan, equipment involved in a power source for supplying electric power to the image forming apparatus has to be subject to special treatment so that the equipment can cover the power supply voltage of 200V. Thus, such an approach to suit power source facilities in offices to the power source voltage 200V may not be a general solution.
Even if an attempt is made to raise the temperature of a heating roller in a short time, the maximum input energy is limited as long as the commercial power source of 100V and 15 A is used. In order to improve this problem, some techniques have been presented.
Japanese Laid-Open Patent Application No. 10-010913 discloses method and device in which the temperature of a fixing device can drop more slowly by supplying a lower voltage to a heating roller during waiting time of the fixing device.
Japanese Laid-Open Patent Application No. 10-282821 discloses method and device in which a secondary cell as an auxiliary power source is charged during waiting time of a fixing device, and when the fixing device is started up, electric power is supplied from the secondary cell or the primary cell together with a main power source device to shorten start up time of the fixing device.
According to the conventional technique disclosed in Japanese Laid-Open Patent Application No. 10-010913, however, since the lower voltage is supplied to the fixing device during waiting time thereof, power consumption of the image forming apparatus can be insufficiently saved. In addition, the technique is not intended to make the maximum supply power at the start up time higher than the level of electric power supplied from the main power source device.
According to the conventional technique disclosed in Japanese Laid-Open Patent Application No. 10-282821, on the other hand, electric power is supplied from the secondary cell or the primary cell together with the main power source device at start up time, and a lead-acid battery, a NiCd battery or a nickel metal hydride battery is used as the secondary cell in general. As such a secondary cell is iteratively charged and discharged, the capacity of the secondary cell is increasingly degraded. Also, as the secondary cell is discharged with a powerful current, the life-span of the secondary cell is shortened. In addition, the capacity of the secondary cell may be reduced due to a so-called “memory effect”. In general, although such secondary cells can supply a large amount of current and have a long life-span, the number of allowable charge-discharge iteration times is about 500 to 1,000. If such a secondary cell is iteratively charged and discharged 20 times a day, the secondary cell comes to the end of the life-span thereof in about one month. Accordingly, it is necessary to replace the battery so frequently, thereby resulting in the corresponding replacement task and increasing in running costs for battery replacement. In addition, a lead-acid battery is not preferred as office equipment in that liquid of sulfuric acid is used in electrolytic solution in the lead-acid battery.
In addition, when supply of a large volume of electric power is started and stopped, drastic current variations and rush current increase a load on a heating circuit in a heating roller. Furthermore, input current is conducted to other circuits in the vicinity of the heating circuit, resulting in noise. For these reasons, it is not preferable to frequently switch ON or OFF electric power supplied from a high-capacity auxiliary power source. Also, when a high-capacity current is quickly supplied to the heating circuit, there is a risk that the heating circuit may be overheated due to excessive supply.
Japanese Laid-Open Patent Application No. 2002-184554 discloses a fixing device in which the above-mentioned problems can be eliminated. The disclosed fixing device can improve power saving efficiency. Also, when a large volume of electric power is supplied, the fixing device can reduce noise caused by rush current and drastic current variations. In addition, the fixing device can not only shorten start up time but also prevent a heating roller from being overheated. The fixing device includes a rechargeable capacitor in an auxiliary power source device thereof. A charger charges the capacitor of the auxiliary power source device by using electric power supplied from a main power source device of the image fixing device. A switch device alternates between charge of the auxiliary power source device and power supply from the auxiliary power source device to an auxiliary heat generation part so as to adjust an amount of electric power supplied to the auxiliary heat generation part. In the fixing device, the capacitor has some functions. The first function is to heat an auxiliary heater by using electric power supplied from the capacitor. The second function is to shorten start up time to raise a heating roller to a predetermined temperature by using the generated heat. The third function is to prevent a fixing temperature from being lowered during passage of a paper.
Although a capacitor has a considerably longer life-span than a battery, iterative charge-discharge shortens the life-span of the capacitor. For example, it is said that an electric double layer capacitor, which has been recently developed, can be iteratively charged and discharged more than 10,000 times. However, it is desirable that a capacitor have a still longer life-span in an image forming apparatus such as a copier, especially an intermittently and repeatedly used image forming apparatus.
In the following, exemplary operation and structure of a conventional electrophotographic type image forming apparatus, such as a copier, a printer and a facsimile, are described. In such an image forming apparatus, typically, a toner image is formed on a paper such as a transferred paper. Then, the toner image is fixed on the paper by heating the toner during passage of the toner adhesive paper through a fixing device.
FIG. 1 shows an exemplary structure of a conventional fixing device.
Referring to FIG. 1, a pressure part (not illustrated) applies predefined nip pressure to a fixing roller 91 via a pressure roller 92. A drive mechanism (not illustrated) revolves the fixing roller 91 clockwise and the pressure roller 92 counterclockwise in terms of the illustration. The fixing roller 91 comprises heaters 93 and 94 as heat generation parts for generating heat by using supplied electric power. The heaters 93 and 94 heat the circumferential surface of the fixing roller 91 to a reload temperature for fixing a toner. Here, a temperature detection part 95, such as a temperature sensor, is in contact with the circumferential surface of the fixing roller 91, and detects the surface temperature of the fixing roller 91.
In image formation of an image forming apparatus having the conventional fixing device, the heated fixing roller 91 and the pressure roller 92 heat a paper P, which supports a toner T thereon in an electrophotographic manner, and fix the toner T on the paper P during passage through a nip part between the fixing roller 91 and the pressure roller 92. In order to properly fix the toner T on the paper P, a predetermined amount of heat has to be applied to the toner T. Accordingly, an amount of electric power supplied to the heaters 93 and 94 is controlled to keep the circumferential surface of the fixing roller 91 at a reload temperature.
FIG. 2 shows an exemplary circuit structure of the conventional fixing device shown in FIG. 1.
Referring to FIG. 2, the heater 93 generates heat by using electric power supplied from an external power source (commercial power source) 87. On the other hand, the heater 94 generates heat by using electric power supplied from a capacitor 88 as an embodiment of an electricity storage device. When a temperature detection part 95 detects the temperature of the fixing roller 91, the detected temperature is supplied as a detection signal to CPU (Central Processing Unit) 83 via an input circuit 82. Based upon the detection signal from the temperature detection part 95, CPU 83 controls an amount of current carried to the heater 93 via a driver 84 and a switch 86 as well as an amount of current carried to the heater 94 via the switch 85 so that the surface temperature of the fixing roller 91 can be regulated to a predefined temperature. It is noted that the capacitor 88 is connected to a charge device 89 and becomes chargeable by switching of the switch 85.
FIG. 3 shows exemplary relations among power supplying time, supply power quantities and fixing roller temperatures of a conventional fixing device.
Referring to FIG. 3, when the fixing device 90 is started up with an idle status thereof, for example, by switching ON the main power source thereof, the heating roller 91 is heated to a reload temperature rapidly by carrying electricity to the heaters 93 and 94 so as to shorten waiting time until the fixing device 90 is made available. Also, if the fixing device 90 is in a steady status after reaching to the reload temperature, the temperature of the fixing roller 91 is maintained by carrying electricity to only the heater 93, as referred to in Japanese Laid-Open Patent Application No. 2002-174988.
In a conventional power supply method, however, even if an electricity storage device is charged from an external power source and a sufficient amount of electric power is already stored therein, electric power is supplied from both of the external power source and the electricity storage device to the heater 94.